Wave transmission system



March 21, 1944. R. D. KIRKLAND WAVE TRANSMISSIQN SYSTEM Filed Feb. 6,1942 FIG.2.

' INVENTOR. .5 fi/PAA 4ND A TTORNEY.

Patented Mar. 21, 1944 WAVE TRANSMISSION SYSTEM Robert D. Kirkland, PortWashington, N. Y.,. assignor to Mackay Radio and Telegraph Company, NewYork, N. Y., a corporation of Delawar 8 Application February 6, 1942,Serial No. 429,131

3 Claims.

This invention relates to wave transmission lines, and particularly tolines for transmitting wave energy to-branch lines leading to waveantennae..

The object of the invention is to prevent Wave reflections in atransmission line supplying energy to branch lines which containstanding waves.

It sometimes occurs that a transmission line is terminated by a pair ofbranch lines joined toether at the end of the transmission line. Such acondition may exist, for example, when a pair of Wave antennae are to besupplied with energy from the transmission line. It is well known thatin order to prevent undesirable wave reflections in the transmissionline, it should be teranimated in an impedance equal to its surgeimpedance. But if the branch lines to which the transmission line isconnected do not provide the required surge impedance, as for examplewhen supplying antennae, they do not prevent the re flections. I havefound that if the standing waves in the branch lines he reduced so thatthe ratio of the maximum to the minimum standing wave current has adefinite value which depends on the relationship of the surge impedanceof the branch lines to the surge impedance of thetransmission line,there will be no substantial reflections in the transmission line. Inthe usual case, wherein the branch lines have the same surge impedanceas the transmission line, I have found that the proper ratio of maximumto minimum standing wave current is two-to-one; In accordance with myinvention, there are provided building-out sections proportioned andconnected. at the proper points of the branch lines toprovide thedesired ratio of maximum to minimum standing wave current in theseconnecting 1ines',5and a consequent substantial absence of wave;reflections in the main transmission line.

This invention will be understood from the following detaileddescription of a preferred embodiment and the accompanying drawing, inwhich Fig. 1 illustrates a diagram of an antenna array supplied by linesaccording to this invention; and.

Fig. 2 is a graph showing a standing wave form obtained in the practiceof. this invention.

Referring to Fig. 1, wave energy is transmitted from a source S along atransmission line composed of conductors I and 2, which may be parallelconductors as shown; or, if desired, the well-known coaxial form of linemight Joe-used if provision be made for transforming from its unbalancedcharacter to a balanced, character. Theother end of the transmissionline is. connected at junction 3, 4. to respective branch lines 5,. 6,and. l, 8. Eachof the branch lines istern1inated in a load, which. in.the illustrated system is aLWaVeJantennaJ Theibranch 5,. 611s connectedat 9, I0 by antenna: conductors II, I2 and branch I, 8 is connected atI3, M to antenna I51, I6; Ordinarily conductors 5 and 1, and 1ilewise, 6and 8, are conductors placed substantially per-- pendicular toconductors I, 2, and antenna I5", i6 is placed below and parallelto'antenna II I2. The conductors 5, "I and 6, 8, when supplying anantenna as shown, are preferably placedquite close together, forexample; twelve inches apart; so that their separation is only asmallfraction of a wavelength, thereby making only a small separation at themiddle of each of the two an tennaeII,I2andI5,IB. k

The invention is not necessarily limited to any particular dimensions ofa load or antenna ar ray, but dimensions which have been foundsatisfactory in an antenna curtain of the type illustrated are:dimension a equal to 0.65? and dimensions 27 equal to 0.6x where A isthe length of the wave to be used.

When the wave energy is supplied by the transmission line I, 2, standingwaves are set up in the antenna conductors in a well-known: manner toproduce the required electromagnetic wave radiation. The conductors 5, 6act as two branch lines joined at the end of the main transmission lineI, 2 for connecting the transmission line with the respective antennaconductors. For aspacing of 0.6x between the upper and lower conductors,each branch line will be 0.3x in length, and there will be standingwaves set up in conductors 5, I5 and I, 8 of these branch lines. It isdesired that these standing waves be excluded from the main-transmissionline I, 2,. which wouldrequire that the impedance at terminals 3,4 ofthe transmission line be made equal to the surgei-mpedance of that-line.As the-impedance of. the an tenna array and its branch lines will notordinarily be equal to the surge impedance, wave re-' fiections willoccur which will produce the undesired standing waves in thetransmissionline.

In order to eliminate the standing waves inthe transmission line I, 2,the impedance due to two branch lines at the junction 3, 4, must be madeequal to the surge impedance of line I',.2, To accomplish this theremust be left in each branch line a standing wave, of a definite ratio ofmaximum to minimum current in the wave. I have found that in theordinary case,. wherein the surge impedance of the main transmissionlines and branch lines are equal the required ratio-of maximum tominimum standing. wave current'is two to one. I have discovered thatthedesired amplitude of standing wave can be obtained by connecting areactance of a proper valueto annul sufficient of the branch linereactance at a point of each branch line where. the. resistance is;ofthe right value, to leave the desired standing wave. I accomplish thedesired correctionby connecting building-out linesacross each branchline. These building-out lines may be composed of parallel conductorsI1, I8, for one line, and I9, 20, for the other line. The conductors:may be made of the same diameter as the conductors of the branch linesand are preferably, although not necessarily, coplanar with conductors5, 6 and 1, 8. The building-out lines may be closed-ended as shown at 2|and 22, respectively, although they might in some cases be leftopen-ended if desired.

The efiect of the use of the building-out lines of the standing wave isillustrated in Fig. 2, wherein the abscissa D represents distance alongthe branch line between the junction 3, I and 9, II) (or I3, I4) and theordinate I represents the standing wave current. The curve W1illustrates the form of the standing wave in the absence of thebuilding-out sections showing that the maximum standing wave current inthe branch line, which will exist at some point (11, is much greaterthan the minimum standing wave current which will exist at some otherpoint (22. When the building-out sections are connected, this standingwave form of current is modified, and by connecting them at the properplaces, the ratio of maximum (I max.) to minimum (I min.) standing wavecurrent may be reduced to its desired value, as shown by the curve W2.

The manner of proportioning and locating the building-out linesaccording to this invention will become apparent from the followingmanner of locating and proportioning them in the system shown, which isgiven as an example: The impedance of each branch line at its point ofconnection with the load (in this case the antenna) may be ascertainedby measurement; and such measurements will give the impedance at points9, I and I3, I4, respectively. From the values it will be possible toascertain the magnitude of the resistance R and reactance X at variouspoints along each of the branch lines 5, 6 and I, 8, prior to theconnection of any building-out section; and for convenience, the valuesmay be plotted against distance along the branch lines to give smoothcurves of the variations. In order to eliminate standing-waves on thetransmission line I, 2, the impedance presented by the branch lines atjunction 3, 4 must be made equal to the surge impedance Z0 of thetransmission line. In the example shown, the characteristic imped ancesof the transmission line and the two branch lines are assumed to be thesame, as this is the most ordinary situation. If these characteristicimpedances are, for example, five hundred ohms each, the impedance ofeach branch line must appear as one thousand ohms resistance, andwithout reactance, at the junction. Computations can then be made in awell-known manner to compute the resistance and reactance componentswhich each branch line must have at various points along its length inorder that its impedance, appearing at the junction 3, 4, shall be 2Z0,that is, 1000-1-10 in the present example. For convenience, these valuesof the required resistance and reactance may be plotted against distancealong the branch lines on the same sheet as the actual values ofresistance and reactance are plotted. At some point the requiredresistance will be found equal to the actual resistance there,- and thisis the point at which the building out section should be connected. Thispoint on each branch line is shown in Fig. l at the distance C from thejunction.

'It remains to proportion the building-out section, or correctiveimpedance, to have the proper reactance to cancel some of the reactanceappearing at the points of connection of the buildingout section. Thebuilding-out section should have a reactance equal to the differencebetween the required reactance at that point and the actual reactance.For example, if the actual reactance of the branch line at theconnection point is found to be of a higher capacitive value than thecapacitive value required there, the buildingout section should beproportioned to introduce an inductive reactance of the value sufiicientto reduce the actual capacitive value down to the required value.Knowing the desired reactance the length 0 of the building-out linerequired to produce it can readily be ascertained in a well-knownmanner. Since some reactance is left in each branch line there willremain therein, standing waves. In the example just given wherein thecharacteristic impedance of all the lines are the same, the requiredratio of maximum (I max.) to minimum (I min.) standing wave current ineach branch line is I max. IF. Although the proper dimensions andlocation of the building-out sections are susceptible of accuratecalculation, it is possible to arrive at the correct place of connectioneven if it had not been at first quite accurately placed owing to slightdiscontinuities or irregularities in the installation. This may be doneby trying out the correcting impedance at different positions along thebranch line until the desired standing wave ratio is produced. Theproper points for connecting the building-out sections might beascertained, for example, by the use of an ammeter or voltmeter whichmaybe placed at difierent points along the connecting line 5, 6, or I, 8.The most convenient ammeter or voltmeter to use is a type which does notrequire the line to be broken to take the reading, but which needs onlyto be coupled to one of the line conductors, and may thus readily bemoved along the line to take meter readings at several points. Thismethod of reading is indicated in Fig. l by meter A shown by the dottedcircles and shown connected at points of conductor 8. When theconnection from this meter is moved along conductor 8 to otherpositions, as shown, the meter readings will change in the generalmanner of the standing wave current curves shown in Fig. 2, so that boththe maximum and. the minimum readings may be readily ascertained on themeter for any point of connection of the, building-out section. If it befound that the ratio I max.

I min.

is not the desired value when the building-out sections are firstconnected, their points of connection may be moved to a different placealong conductors 5, 6, and I, 8, and the ratio I max.

I min. again determined by running the meter along the conductor. Inthis way the proper points of connection of the building-out sectionscan easily be established.

If more convenient, the meter may be moved along the transmission lineI, 2, instead of along the branch line. The position of the buildingoutsections for which there is the least or no change in meter readingalong the line I, 2, is

the correct position. No change of meter reading would indicate theperfect condition of no standing waves in the transmission line, and theproper standing wave ratio in the branch line.

Although the example given above has been particularly applied to aclosed-ended, buildingout section giving an inductive correctingreactance, it will be understood that there may be some conditions underwhich an open-ended building-out section might be used, as where acapacitive correcting reactance is desired. Such capacitive building-outlines would be connected to places where the branch line is inductive.When the branch lines are used to feed antenna arrays of the type shownin the example, an inductive section of the line to which thebuilding-out section may be connected is not so likely to be present asa suitable capacitive section of the branch line, as the branch linewill usually be a capacitive reactance extending from the antennapractically to the junction with the transmission line I, 2.

The building-out sections are coplanar with the branch line, this beinga convenient form wherein the building-out line may readily be fastenedto the branch line since the conductors are parallel.

The invention is not necessarily limited to the use of branch lines ofthe same surge impedance as the transmission line, wherein the desiredstanding wave ratio in the branch lines is two to-one, although that isthe most common situation. If the surge impedances of the branch linesare different from the transmission line, the standing wave ratio neededin the branch lines to prevent wave reflections at their junction withthe main line, will be somewhat differ ent from two-to-one; but therequired locations and dimensions of the building-out sections can becalculated from knowledge of the impedance of the branch lines, just asin the above example.

What is claimed is:

1. In a wave transmission system, a transmission line, a pair of branchlines in which they are created standing waves, joined to the end ofsaid transmission line, each of said lines having the same surgeimpedance, and a corrective impedance connected across each of saidbranch lines at a place which causes the ratio of the maximum to theminimum standing wave current in each branch line to becomesubstantially two-to-one, said corrective impedance comprisingconductors lying in the same plane as and between the conductors of thebranch line to which it is connected.

2. In a system for transmitting and radiating electromagnetic waves, anantenna array comprising two parallel antenna conductors, each parted atthe middle, a connecting line comprising two conductors extendiingparallel to each other between the respective parted ends of the twoantenna conductors, a transmission line connected to the middle of saidconnecting line for supplying wave energy which produces standing wavesin the antenna and connecting line, said transmission line andconnecting line having the same surge impedance, and a pair ofbuildingout lines connected across said connecting line, one on eachside of the junction with said transmission line, each of saidbuilding-out lines comprising a pair of parallel conductors lying in thesame plane as and between the conductors of said connecting line, theproportions and points of connecting of said building-out lines beingfixed to cause the ratio of the maximum to the minimum standing wavecurrent in the connecting line to become substantially two-to-one.

3. A system according to claim 2, in which the conductors of theconnecting line and the respective building-out lines parallel therewithare equally spaced in the plane.

ROBERT D. KIRKLAND.

